The GTK+ Drawing Model

The GTK+ Drawing Model — The GTK+ drawing model in detail

Overview of the drawing model

This chapter describes the GTK+ drawing model in detail. If you are interested in the procedure which GTK+ follows to draw its widgets and windows, you should read this chapter; this will be useful to know if you decide to implement your own widgets. This chapter will also clarify the reasons behind the ways certain things are done in GTK+; for example, why you cannot change the background color of all widgets with the same method.

Programs that run in a windowing system generally create rectangular regions in the screen called windows. Traditional windowing systems do not automatically save the graphical content of windows, and instead ask client programs to repaint those windows whenever it is needed. For example, if a window that is stacked below other windows gets raised to the top, then a client program has to repaint the area that was previously obscured. When the windowing system asks a client program to redraw part of a window, it sends an exposure event to the program for that window.

Here, "windows" means "rectangular regions with automatic clipping", instead of "toplevel application windows". Most windowing systems support nested windows, where the contents of child windows get clipped by the boundaries of their parents. Although GTK+ and GDK in particular may run on a windowing system with no such notion of nested windows, GDK presents the illusion of being under such a system. A toplevel window may contain many subwindows and sub-subwindows, for example, one for the menu bar, one for the document area, one for each scrollbar, and one for the status bar. In addition, controls that receive user input, such as clickable buttons, are likely to have their own subwindows as well.

Generally, the drawing cycle begins when GTK+ receives an exposure event from the underlying windowing system: if the user drags a window over another one, the windowing system will tell the underlying window that it needs to repaint itself. The drawing cycle can also be initiated when a widget itself decides that it needs to update its display. For example, when the user types a character in a GtkEntry widget, the entry asks GTK+ to queue a redraw operation for itself.

The following sections describe how GTK+ decides which widgets need to be repainted, and how widgets work internally in terms of the resources they use from the windowing system.

Window and no-window widgets

A GdkWindow represents a window from the underlying windowing system on which GTK+ is running. For example, on X11 it corresponds to a Window; on Win32, it corresponds to a HANDLE. The windowing system generates events for these windows. The GDK interface to the windowing system translates such native events into GdkEvent structures and sends them on to the GTK layer. In turn, the GTK layer finds the widget that corresponds to a particular GdkWindow and emits the corresponding event signals on that widget.

When the program needs to redraw a region of a GdkWindow, GDK generates an event of type GDK_EVENT_EXPOSE for that window. The GTK+ widget layer in turn finds the widget that corresponds to that window, and emits the expose-event signal for that widget.

In principle, each widget could have a GdkWindow of its own. With such a scheme, the drawing cycle would be trivial: when GDK notifies the GTK layer about an exposure event for a GdkWindow, the GTK layer would simply emit the expose-event signal for that widget. The widget's expose event handler would subsequently repaint the widget. No further work would be necessary; the windowing system would generate exposure events for each window that needs it, and then each corresponding widget would draw itself in turn.

However, in practice it is convenient to have widgets which do not have a GdkWindow of their own, but rather share the one from their parent widget. Such widgets have called gtk_widget_set_has_window to disable it; this can be tested easily with the gtk_widget_get_has_window() function. As such, these are called no-window widgets.

No-window widgets are useful for various reasons:

Some widgets may want the parent's background to show through, even when they draw on parts of it. For example, consider a theme that uses textured backgrounds, such as gradients or repeating patterns. If each widget had its own window, and in turn its own gradient background, labels would look bad because there would be a visible break with respect to their surroundings. Figure 1, “Windowed label vs. no-window label” shows this undesirable effect.

Figure 1. Windowed label vs. no-window label
Reducing the number of windows creates less traffic between GTK+ and the underlying windowing system, especially when getting events.

On the other hand, widgets that would benefit from having a "hard" clipping region may find it more convenient to create their own windows. Also, widgets which want to receive events resulting from user interaction may find it convenient to use windows of their own as well. Widgets may have more than one window if they want to define different regions for capturing events.

Hierarchical drawing

When the GTK layer receives an exposure event from GDK, it finds the widget that corresponds to the window which received the event. By definition, this corresponds to a widget that has the GTK_NO_WINDOW flag turned off (otherwise, the widget wouldn't own the window!). First this widget paints its background, and then, if it is a container widget, it tells each of its GTK_NO_WINDOW children to paint themselves. This process is applied recursively for all the GTK_NO_WINDOW descendants of the original widget.

Note that this process does not get propagated to widgets which have windows of their own, that is, to widgets which have the GTK_NO_WINDOW flag turned off. If such widgets require redrawing, then the windowing system will already have sent exposure events to their corresponding windows. As such, there is no need to propagate the exposure to them on the GTK+ side.

Figure 2, “Hierarchical drawing order” shows how a simple toplevel window would paint itself when it contains only GTK_NO_WINDOW descendants:

The outermost, thick rectangle is a toplevel GtkWindow, which is not a GTK_NO_WINDOW widget — as such, it does receive its exposure event as it comes from GDK. First the GtkWindow would paint its own background. Then, it would ask its only child to paint itself, numbered 2.
The dotted rectangle represents a GtkVBox, which has been made the sole child of the GtkWindow. Boxes are just layout containers that do not paint anything by themselves, so this GtkVBox would draw nothing, but rather ask its children to draw themselves. The children are numbered 3 and 6.
The thin rectangle is a GtkFrame, which has two children: a label for the frame, numbered 4, and another label inside, numbered 5. First the frame would draw its own beveled box, then ask the frame label and its internal child to draw themselves.
The frame label has no children, so it just draws its text: "Frame Label".
The internal label has no children, so it just draws its text: "This is some text inside the frame!".
The dotted rectangle represents a GtkHBox. Again, this does not draw anything by itself, but rather asks its children to draw themselves. The children are numbered 7 and 9.
The thin rectangle is a GtkButton with a single child, numbered 8. First the button would draw its beveled box, and then it would ask its child to draw itself.
This is a text label which has no children, so it just draws its own text: "Cancel".
Similar to number 7, this is a button with a single child, numbered 10. First the button would draw its beveled box, and then it would ask its child to draw itself.
Similar to number 8, this is a text label which has no children, so it just draws its own text: "OK".

Figure 2. Hierarchical drawing order

To avoid the flickering that would result from each widget drawing itself in turn, GTK+ uses a double-buffering mechanism. The following sections describe this mechanism in detail.

Notes on drawing no-window widgets

Remember that the coordinates in a GdkEventExpose are relative to the GdkWindow that received the event, not to the widget whose expose-event handler is being called. If your widget owns the window, then these coordinates are probably what you expect. However, if you have a GTK_NO_WINDOW widget that shares its parent's window, then the event's coordinates will be offset by your widget's allocation: remember that the allocation is always relative to the parent window of the widget, not to the parent widget itself.

For example, if you have a no-window widget whose allocation is { x=5, y=6, width, height }, then your drawing origin should be at (5, 6), not at (0, 0).

Drawing over child windows

When you draw on a GdkWindow, your drawing gets clipped by any child windows that it may intersect. Sometimes you need to draw over your child windows as well; for example, when drawing a drag-handle to resize something. In this case, turn on the GDK_INCLUDE_INFERIORS subwindow mode for the GdkGC which you use for drawing.

Double buffering

When the GTK layer receives an exposure event from GDK, it first finds the !GTK_NO_WINDOW widget that corresponds to the event's window. Then, it emits the expose-event signal for that widget. As described above, that widget will first draw its background, and then ask each of its GTK_NO_WINDOW children to draw themselves.

If each of the drawing calls made by each subwidget's expose-event handler were sent directly to the windowing system, flicker could result. This is because areas may get redrawn repeatedly: the background, then decorative frames, then text labels, etc. To avoid flicker, GTK+ employs a double buffering system at the GDK level. Widgets normally don't know that they are drawing to an off-screen buffer; they just issue their normal drawing commands, and the buffer gets sent to the windowing system when all drawing operations are done.

Two basic functions in GDK form the core of the double-buffering mechanism: gdk_window_begin_paint_region() and gdk_window_end_paint(). The first function tells a GdkWindow to create a temporary off-screen buffer for drawing. All subsequent drawing operations to this window get automatically redirected to that buffer. The second function actually paints the buffer onto the on-screen window, and frees the buffer.

Automatic double buffering

It would be inconvenient for all widgets to call gdk_window_begin_paint_region() and gdk_window_end_paint() at the beginning and end of their expose-event handlers.

To make this easier, most GTK+ widgets have the GTK_DOUBLE_BUFFERED widget flag turned on by default. When GTK+ encounters such a widget, it automatically calls gdk_window_begin_paint_region() before emitting the expose-event signal for the widget, and then it calls gdk_window_end_paint() after the signal has been emitted. This is convenient for most widgets, as they do not need to worry about creating their own temporary drawing buffers or about calling those functions.

However, some widgets may prefer to disable this kind of automatic double buffering and do things on their own. To do this, turn off the GTK_DOUBLE_BUFFERED flag in your widget's constructor.

Example 1. Disabling automatic double buffering

static void
my_widget_init (MyWidget *widget)
{
  ...

  gtk_widget_set_double_buffered (widget, FALSE);

  ...
}

When is it convenient to disable double buffering? Generally, this is the case only if your widget gets drawn in such a way that the different drawing operations do not overlap each other. For example, this may be the case for a simple image viewer: it can just draw the image in a single operation. This would not be the case with a word processor, since it will need to draw and over-draw the page's background, then the background for highlighted text, and then the text itself.

Even if you turn off the GTK_DOUBLE_BUFFERED flag on a widget, you can still call gdk_window_begin_paint_region() and gdk_window_end_paint() by hand to use temporary drawing buffers.

App-paintable widgets

Generally, applications use the pre-defined widgets in GTK+ and they do not draw extra things on top of them (the exception being GtkDrawingArea). However, applications may sometimes find it convenient to draw directly on certain widgets like toplevel windows or event boxes. When this is the case, GTK+ needs to be told not to overwrite your drawing afterwards, when the window gets to drawing its default contents.

GtkWindow and GtkEventBox are the only two widgets which will draw their default contents unless you turn on the GTK_APP_PAINTABLE widget flag. If you turn on this flag, then they will not draw their contents and let you do it instead.

The expose-event handler for GtkWindow is implemented effectively like this:

static gint
gtk_window_expose (GtkWidget      *widget,
		   GdkEventExpose *event)
{
  if (!gtk_widget_get_app_paintable (widget))
    gtk_paint_flat_box (widget->style, widget->window, GTK_STATE_NORMAL, 
                        GTK_SHADOW_NONE, event->area, widget, "base", 0, 0, -1, -1);

  if (GTK_WIDGET_CLASS (gtk_window_parent_class)->expose_event)
    return GTK_WIDGET_CLASS (gtk_window_parent_class)->expose_event (widget, event);

  return FALSE;
}

The expose-event handler for GtkEventBox is implemented in a similar fashion.

Since the expose-event signal runs user-connected handlers before the widget's default handler, what happens is this:

Your own expose-event handler gets run. It paints something on the window or the event box.
The widget's default expose-event handler gets run. If GTK_APP_PAINTABLE is turned off (this is the default), your drawing will be overwritten. If that flag is turned on, the widget will not draw its default contents and preserve your drawing instead.
The expose-event handler for the parent class gets run. Since both GtkWindow and GtkEventBox are descendants of GtkContainer, their no-window children will be asked to draw themselves recursively, as described in the section called “Hierarchical drawing”.

Summary of app-paintable widgets. Turn on the GTK_APP_PAINTABLE flag if you intend to draw your own content directly on a GtkWindow and GtkEventBox. You seldom need to draw on top of other widgets, and GtkDrawingArea ignores this flag, as it is intended to be drawn on.